The present invention relates to a telecommunication system comprising a main station and a plurality of substations, in which system communication between the main station and the substations occurs via a transmission channel on the basis of a multiple access protocol, which channel is at least partially common to the substations. For coarse ranging the substations are provided with ranging transmission means for transmitting a low magnitude ranging sequence with respect to the magnitude of data to be transmitted in time slots, and the main station is provided with correlation means for recovering ranging information by correlating a received ranging sequence with a reference sequence which is identical to the transmitted sequence. The main station is provided with means for providing the reference sequence. Depending on the transmission channel used, i.e. a glass fibre, a coaxial cable or a radio link, the telecommunication system can be a passive optical network (PON), a local area network, a satellite communication system or a cellular mobile radio system.
The present invention further relates to a main station for use in such a system.
A telecommunication system of this kind is known from the article, "Burst and bit synchronization methods for passive optical networks", C. A. Eldering et al., 4th Workshop on Optical Local Networks, IEEE, Sep. 24, 25 1992, Versailles, France, pp. 38-44. In the article various synchronization techniques are described, such as burst synchronization or coarse ranging, such synchronization being necessary to prevent substations from interfering with each other due to non-synchronized operation, i.e. to prevent data transmitted from the substations to the main station from arriving at the main station simultaneously. In the known telecommunication system, an optical network, the main station may transmit data and messages in time slots within a frame to the substations, in the downstream direction, in a controlled way, i.e. the main station or master has full control over the timing of the data to be transmitted to the various substations. In the upstream direction, from the substations to the main stations the most common technique is a Time Division Multiple Access Technique CFDMA), which requires that the delays between the substations and the main station be known and that a suitable delay be incorporated in the transmission from each substation so that the burst transmissions arrive at the specified or allocated time slots for the substations with respect to the main station timing. Such TDMA techniques can be used in combination with a frame structure or without a frame structure, the latter as ATM (Asynchronous Transfer Mode) cells to be transported using a TDMA protocol. Especially when no frame structure is present it becomes essential to have a non-intrusive coarse ranging for the ATM transport on fiber, but also with a frame structure a non intrusive coarse ranging method can be used, i.e. a method which practically does not interfere with the transmissions from in-service substations. On page 41 of said IEEE article a non intrusive coarse ranging technique is disclosed based upon correlation techniques. The known coarse ranging technique, i.e. ranging with an accuracy of a few data bits, is based on correlation of transmitted and received sequences, and estimates the range between the main station and the substation of an out-of-service substation while the other substations may continue transmission of information. The substation to be ranged transmits a long ranging sequence with very low magnitude and at a lower speed than transmission of data (e.g. in a TDMA ATM architecture a bit period of the ranging sequence corresponds to a cell period). A sequence with good correlation properties is chosen, the low power being necessary to prevent disturbance to the data to be disturbed by the sequence, which behaves like a Pseudo Noise Sequence. The ranging sequence can be a maximum length sequence with finite length (2.sup.n -1 ), i.e. a sequence obtainable with a linear feedback shift register or a linear sequence generator, described in numerous handbooks on Logic Design. At the main station side, the ranging information can be recovered from the received ranging sequence using correlation techniques. Therefore, the received signal is sampled in the main station's receiver and correlated with the transmitted, i.e. known, maximal length sequence. In FIG. 6, page 41 of said IEEE article for this purpose a correlation receiver is disclosed, an output of which gives an immediate indication of the delay between the main station and the substation to be ranged. Apart from mentioning that experiments were carried out with maximum length sequences of 15 to 127 bits in this coarse ranging system, no implementation is disclosed with respect to the correlator. The correlation has been described in relation to TDMA ATM having no frame structure. The IEEE-article further discloses coarse ranging techniques with respect to a TDMA based PON having a frame structure, these techniques being based upon recognition in the main station of ranging bits to be transmitted in preambles of bursts by the substations. Because of the fact that the magnitude of the ranging bits is in the same order as the magnitude of the data to be transmitted by other substations, a ranging window is provided with the main station's frame to allow substations which are coming into service to transmit bursts for coarse ranging purposes.
In the, PCT Application WO 91/06157 a passive optical network is disclosed with a frame based TDMA communication protocol in which network upstream TDMA times are transmitted from the substations to the main station with ranging pulses for coarse ranging and ranging pulses for fine ranging to compensate for different delays associated with the substations with respect to the main station. As compared with the correlation technique disclosed in said IEEE-article with respect to the frameless TDMA ATM system, extra bits are necessary for coarse and fine ranging, thus decreasing frame efficiency.
It is an object of the present invention to provide a more accurate coarse ranging correlation technique for an operational telecommunication system.
To this end a telecommunication system according to the present invention is characterized in that the communication is frame based, and in that for coarse ranging the correlation means only correlate samples of the received ranging sequence substantially falling: inside a time window reserved in the frame for ranging. The present invention is; based upon the insight that in a system in operation, operational substations are monitored by the main station in a ranging window, preferably a fine ranging window, in which window no data interference is present, the ranging sequence in principle being transmitted during a complete upstream multiframe. This time window, the time position of which is known to the main station and preferably being reserved for fine ranging, is used during coarse ranging for acquiring samples to be correlated with the reference sequence, the samples thus substantially being free of interference with data from other substations already in operation. Consequently, the signal to interference ratio of the received coarse ranging signal is an order of magnitude better than in other parts of the frame. Due to the improved signal to interference ratio the phase of the received ranging sequence with respect to the reference sequence can be determined more accurately as compared with a low magnitude ranging sequence having the same magnitude in a system where correlation samples would be acquired at a random position within the frame. In such a system the correlation process would be subject to low frequency components of the data from other substations and to receiver noise. Although the ranging sequence can be operated at the data bitrate, for practical reasons it is advantageous to apply a ranging sequence bitrate which is fraction of the data bitrate, e.g. 0.1, in order to avoid high sampling rates within the system. The magnitude of the coarse ranging sequence in the system according to the invention can be reduced with respect to a sequence sampled in a randomly chosen window. In the latter case the still lower magnitude ranging sequence interferes less with the data of operational substations, giving overall better system performance. In the case of a telecommunication system with one kind of ranging, such as a mobile radio system, the ranging according to the present invention is applied by sampling in a reserved window only.
In an embodiment of the system according to the present invention, the bitrate of the ranging sequence is lower than the data bitrate of the substations, and in addition to correlation of samples of the received ranging sequence inside the time window, the correlation means correlate samples of the received ranging sequence outside the time window, substantially more samples being taken inside the time window than outside the time window. In this embodiment more samples are available for correlation so that the phase of the received sequence can be determined more accurately.
In a further embodiment of a system according to the present invention, the main station comprises a low-pass filter for filtering the received ranging sequence before it is fed to the correlation means, the low-pass filter having a cut-off frequency substantially coinciding with the frequency of the received ranging signal. If the ranging sequence bitrate is lower than the data bitrate, higher frequency components of the data are suppressed, improving system performance due to an increased signal to interference ratio for coarse ranging samples outside the fine ranging window. In fact, the latter is a combined use of a selectively chosen correlation window as of the present invention and an out-of-band signalling technique. Out-of-band signalling techniques are known per se from the said IEEE article, page 40.